It is well known that heat will destroy microorganisms. The presence of moisture accelerates this destruction by denaturing or coagulation of the protein making up the microorganism. The importance of moist heat in the sterilization process has been known for about 100 years. While Pasteur had established the fact that temperatures above the boiling point of water were required to kill many organisms, it was not until 1880 that a steam pressure sterilizer was developed. However, there was no significant notice of the need for sterilization of hospital equipment and fabrics until the introduction of first antiseptic and then aseptic surgery.
About the turn of the century manually controlled sterilizing autoclaves were introduce into hospitals. Their efficiency left much to be desired, but nonetheless, this was a definite step forward. As a result of advances in the food industry a fuller understanding of parameters required for spore kill were developed, and it was determined that the order of death was logarithmic.
The most significant aspect in the development of safe and effect sterilization techniques was the recognition of the presence of moisture in the killing of microorganisms. Most microorganisms contain sufficient moisture so that moderate heat alone, e.g., 80.degree.-100.degree. C., will destroy the microorganism. Most bacterial spores, however, contain substantially no moisture. Their destruction by dry heat alone requires elevated temperatures in excess of 150.degree. C. Such excessive temperatures can result in the destruction of the article to be sterilized, or otherwise seriously limit its useful life. Hence, in the hospital arena pathogenic spores are destroyed using a steam atmosphere In autoclaves.
While the first effective steam sterilizers were conventionally operated at about 250.degree. F. for sterilization cycles of about 12 to 15 minutes, the preferred sterilization conditions, presently, are about 270.degree. F. for about 3 minutes.
Lacking any adequate means of monitoring the process, to ensure an adequate safety margin, sterilization times as long as thirty minutes have been used to ensure that 100% of the pathogenic spores have been killed. Such long sterilization cycle times give the operator a degree of confidence that steam has penetrated throughout the autoclave and among all of its contents. However, long heating cycles are disadvantageous from the stand point of economy of time, energy consumption and deleterious effects of the materials to be sterilized, e.g., fabric gowns, drapes, muslin products, certain plastic devices, etc. For these and other reasons there was a continual attempt to develop methods to confidently monitor the sterilization process so that time exposure could be minimized.
Initial attempts at monitoring the steam sterilization process relied on chemical type indicators. The crudest variety comprise a sealed tube containing a compound with a melting point which corresponds to the sterilization temperature to be achieved. One of the earliest of such devices was sold under the name DIACK.TM.. See for example U.S. Pat. Nos. 3,313,266; 3,341,238 and 3,652,249. These devices are capable of doing no more than indicating whether or not the autoclave has reached the melting point of the chemical in the lube for a time sufficient to melt the chemical.
Other sterilization process monitors rely on a temperature accelerated reaction to cause color change in an indicator. Though some of these devices purport to be operative at more than one temperature/time condition, they suffer from the disadvantage that they do not match the spore kill temperature/time relationship.
The thermal resistance of spores of a particular species at any temperature is characterized by its temperature coefficient. The symbol Q.sub.10 is used to designate the temperature coefficient over a range of 10.degree. C., and is the ratio of the death rate constant at a particular temperature to the death rate constant at a temperature 10.degree. C. lower. Generally, the measurements are made for a fixed time interval, e.g., 9 minutes. If the constants at two temperatures, t.sub.1 and a temperature, t.sub.2, 10.degree. C. higher are known, Q.sub.10 may be calculated from the equation ##EQU1## wherein t.sub.1 and t.sub.2 are as defined and K.sub.1 and K.sub.2 are the respective death rate constants.
Where temperature is expressed In degrees Fahrenheit the relationship is described in terms of Z number rather than Q.sub.10. The relationship is described by the following formula: EQU t=F.sub.o .times.10.sup.(250-T) /Z
wherein t is the kill time for spores at T.degree. F and F.sub.o is the kill time for spores at 250.degree. F.
Spores generally exhibit a Q.sub.10 value of about 10[.degree.C.], or a Z number of 18[.degree.F.]. Therefore, it is desirable to have a sterility indicator which will, in a sense, mimic spore kill. To do so, the ratio of the effect of temperature as a function of time on a measurement taken at one temperature as compared to the same measurement at another temperature 10.degree. C. lower should also be 10. To be useful as a sterility indicator, this relationship must also be dependent on the presence of moisture, since the spore kill time/temperature relationship is vastly different in the dry or wet state. In the absence of moisture spore kill at 270.degree. F. is negligible, but in the presence of steam spore kill is virtually complete for the most resistant strains at these temperatures in about 1-2 minutes.
From time to time attempts have been made to develop sterilization indicators which permit quality control of sterilization with the confidence that all micro-organisms have been destroyed. In the past, the most satisfactory method has been the use of spore strips. Spores which are particularly difficult to destroy are selected as the control standard, e.g., Bacillus Subtilis vat., Niger and Bacillus Stearothermophilus. The spore strip is placed in the autoclave with the materials to be sterilized. At the end of the sterilization cycle, the spore strip is studied to determine whether it is possible to grow organisms in a suitable culture medium. Failure of the spores to reproduce indicates death of spores; and hence, adequate sterilization.
Although this control technique is accurate, it suffers from several inherent disadvantages, (1) excessive cost (2) delay between processing and control data (3) batch to batch variation of the spores and (4) heat resistance of spores decreases with storage time.
The first successful chemical type steam sterilization process monitor to mimic spore kill was that of the Larsson U.S. Pat. No. 3,981,683, incorporated herein by reference. This device comprised a backing strip, a chemical compound whose normal melting point was above the sterilization temperature to be monitored, the chemical compound being mounted on the backing strip toward one end thereof, a wicking means in contact with and extending away from the chemical compound toward the distal end of the backing strip, and a cover strip which is rate controlling with respect to the ingress of steam. The chemical compound is selected so that its melting point is depressed by the absorption of water passing through the cover strip in the vapor phase. Although Larsson disclosed that the cover strip could be adhered to the backing by adhesive bonding or heat sealing, no guidance is given with respect heat sealing techniques.
An attempt was made to develop a heat sealed device of the Larsson '683 type device. See U.S. Pat. No. 4,410,493, which issued covering a heat sealed device. Attempts to commercialize the device failed. The patent discloses that the cover layer should be adhered to the backing strip and wicking means by heat sealing thereby enclosing the entire device by full contact of the cover layer with the backing, chemical compound and wicking means.
An improved device of the Larsson type was patented to Foley with the issuance of his U.S. Pat. No. 4,448,548, incorporated herein by reference. The improvements disclosed by Foley were the ability to control the speed of advance of the color front produced by the wicking of the chemical compound. This control is attributed to the selection of an acrylic adhesive and the incorporation into the chemical compound of a binder such as polyvinyl pyrrolidone.
Both Larsson and Foley commercialized their devices by utilizing an adhesive to secure the cover strip to the backing. Until recently these devices have operated successfully in all types of sterilizers including high vacuum units. In conventional sterilization processes the item to be sterilized is wrapped in a fabric wrapping with the sterilization monitor enclosed therein. A unique application for the device has been the monitoring of the sterilization process in the operating room where "flash sterilizers" were used to sterilize materials required immediately during an operation in progress. In this sterilization process the items to be sterilized as well as the sterilization process monitor are laid exposed and unwrapped in the flash sterilizer. No problems had been encountered in using the sterilization monitors in this application until the recent advent of high vacuum flash sterilizers.
The fact that the sterilization monitor lies exposed in the flash sterilizer exposes it directly to the high vacuum and steam. The method of operating the flash sterilizer is to expose the materials to be sterilized to alternating cycles of steam at about 134.degree. C. and high vacuum. The first cycle is a steam cycle followed by a high vacuum cycle. The result is that the monitors of Larsson which are adhesively sealed, are directly exposed to the hot steam resulting in softening of the adhesive. The subsequent high vacuum cycle causes the adhesive, weakened by high temperature exposure, to give way resulting in failure of the device.
What is required is an improved sterilization monitor of the type described which will withstand the adverse conditions of a high vacuum flash sterilizer.